The present invention relates to a scanning method for reducing block effect in image coding, and more particularly to a symmetrical image block scanning method which can achieve better image quality by changing the scanning order.
In general, vector quantisation is known to be an effective scheme for data compression, especially for low-rate image date compression. To achieve low-rate vector quantisation system with acceptable image quality, the vector dimension should be increased. With the dimension increased, block effects become more visible. There has been extensive research directed towards reducing block effects, which are mainly related to transform coding methods. As an example, raster scan method is widely used as the scanning method. In the raster scan method, the scan is conducted in the order of which is shown in FIG. 8, and picture elements arranged in the scanning order constitute one vector which indicates said image block. In FIG. 8, five scan lines are illustrated in one block for convenience, but in fact the number of scan lines may be higher than that or lower than that of said lines.
In such raster scan method, however, the starting points of block scan or the ending points of block scan exist separately by the distance of one block because all of the blocks adjacent to each other are scanned in the same shape. Accordingly, in the process of quantisation of the vectors, the spatial relations among picture elements are not reflected so that among the reproduced image blocks block boundary effects may be produced. Particularly, there is a disadvantage in that the bigger the quantisation interval, the block boundary effects become serious.